1. Field of Invention
The present invention relates generally to controls, and specifically to a simplified method of sensing the direction of net energy flow through a fluid-separation barrier which transmits a radiant energy flux. Particular applications include the control of thermal shutters in industrial processes, solar collectors, and solar space heaters.
2. Prior Art
Heretofore, control of industrial processes and solar collectors by means of thermal gain was not widely employed because of the in-accuracy of simple means and the complexity of accurate means.
One heat transfer problem, well understood in theory, involves a net absorption of radiant energy by the controlled environment on the one hand, and loss of thermal energy to a cooler outside environment on the other. The energy loss results from: heat transfer by convection from the controlled environment to the window surface; heat transfer by conduction through the materials and air spaces comprising the window; and heat transfer by convection (natural and possibly forced as in the case of wind) to the cooler outside environment.
As an example, consider a south facing window used as a solar collector in the winter. The window is fitted with curtains (solar shutters) to reduce heat loss. The problem is when should the curtains be opened.
As a simple approach to this control problem, the curtains are drawn at night and opened in the day. A solar cell based controller might be used to automate the opening and closing. For lack of more accurate information, the occupant has assumed that whenever the sun is shining energy is being gained. A closer consideration, however, shows that thermal losses through a window can easily exceed the radiant gain, depending on the outside temperature, sun angle, cloudiness, and wind. Quite often, the curtains should be drawn even though sun is coming through the window.
For improved collection efficiency, the control method must consider the energy losses out of the window as well as the radiant energy gain into the window. The most difficult loss factor to determine is the convective transfer of the room air to the window. By present methods, this requires sophisticated instrumentation to measure the fundamental quantities and calculate the net energy flow from the governing equations. This includes accurate measurement of: the outside temperature, any "wind" or forced movement of the outside fluid, the thermal conductivities and thicknesses of all window materials, the inside temperature, the absorptivity of the controlled environment, and the net amount of radiation entering the controlled environment. In addition, certain thermal properties of the inside and outside fluids must be known over the operational temperature range, namely: kinematic viscosity, coefficient of volume expansion, density, specific heat, and the thermal conductivity.
An automated control using these measurements requires a sophisticated electronic computer capable of storing tables of empirical fluid characteristics and capable of computing fractional powers of numbers. Further, complex sensors and instrumentations are required on both sides of the barrier. In addition, a new "program" is required for different window compositions and fluids.